Pollution of subsurface groundwater by man-made toxic and carcinogenic compounds is widespread throughout the world. Sources of these pollutants include solvents used in industrial processes, hydrocarbon fuels and dry cleaning chemicals. Often these pollutants are released into the environment from leaking pipes and underground storage tanks, poor materials handling practices, and catastrophic spills. Upon release, the pollutants in liquid and gaseous form migrate down through unsaturated soils and fractured rock, eventually impacting groundwater.
Groundwater Pollutant Migration
A source of groundwater pollution, left unchecked, will produce a plume of dissolved or phase-separated pollutant molecules that will move through the groundwater system or aquifer. In aquifers composed of heterogeneous materials (e.g. sand and clay layers) the pollutant tends to flow primarily through zones of higher permeability (e.g. sands). As the zones of higher permeability transport elevated pollutant concentrations a diffusion gradient is established driving the pollutant into adjacent zones of lower permeability (e.g. clays). Over time this can result in adjacent lower permeability zones storing significant masses of dissolved pollutants.
Groundwater Remediation and Back-Diffusion
Over the past several decades groundwater remediation techniques have evolved from simple groundwater extraction technologies to in situ treatment techniques involving for example the injection of surfactant flushing agents, oxidizing agents, reducing agents, and amendments to stimulate the in-place biodegradation of pollutants. These techniques have been shown to impact mostly the high permeability zones within the aquifer system.
Thus, while historical remediation approaches remove contaminant concentrations from the more permeable zones, zones of lower permeability are less treated. This sets up a reversal in the contaminant diffusion gradient referred to as “back diffusion”, where dissolved contaminant concentrations stored in the lower permeability zones diffuse back into the areas of higher permeability where contaminants have been removed. Back diffusion has been shown to occur over very long periods of time causing persistent low levels of contaminants to impact groundwater wells after attempts at aquifer remediation.
Back Diffusion Stalls Remediation Programs
In a recent report from the National Research Council (NRC, 2012 and references therein), it was estimated that over $200 billion will be spent on cleanup at 300,000 contaminated sites in the U.S. through the year 2033. Some of the key reasons for protracted timeframes and increased costs on these site cleanups are: “difficulties in characterizing the nature and extent of the problem in highly heterogeneous subsurface environments, as well as use of remedial technologies that have not been capable of achieving restoration in many of these geologic settings.” In addition, the document goes on to identify back diffusion (also known as matrix back diffusion) as one of the prominent processes that limit our ability to clean up groundwater at complex sites. The NRC report also states that “there are no proven remedial techniques to preferentially target and accelerate the removal of contaminants from localized sites that are desorption/diffusion limited.” In light of this report there is clearly a need for techniques that address groundwater contamination associated with matrix back diffusion and thus allow faster and lower-cost cleanup of contaminated sites.